How to determine aluminum content of a sample

Chemicals Required

Acetic Acid - Sodium Acetate Buffer Aluminium Standard

Ascorbic Acid Solution

Disodium Ethylenediamine Tetra-acetate (EDTA), 0.01M Eriochrome Cyanine R Dye Indicator

Hydrochloric Acid,

50% Methyl Orange Indicator

Sodium Hydroxide,

1.0N Sulfuric Acid, 0.02N; 6.0N

Procedure for Test

This procedure uses a wavelength of 535 nm and a cell with a path length of approximately 10 mm. A calibration or working curve for the photometer is constructed by using successive dilutions of the stock aluminium solution. Prior to performing the test, all glassware should be rinsed thoroughly with warm 50 per cent HCl and washed copiously with aluminium free distilled water to avoid contamination.

Preparation of Calibration Curve

To a 25-ml volumetric flask, add the appropriate amount of aluminium standard needed to adequately cover the anticipated range of aluminium in the sample to be tested. Add distilled water to bring the volume to approximately 12 ml; then add 0.5 ml of 0.02N Sulphuric acid and mix. Next add 0.5 ml of ascorbic acid solution and again mix. Add 5 ml of buffer solution, mix, and add

2.5 ml (accurately with a pipet) of the Eriochrome cyanine R dye indicator. Next bring the total volume in the flask to 25 ml by adding distilled water. Transfer this solution to a photometer cell and read the absorbance after five minutes. Do not wait longer than 15 minutes for this reading as the color then begins to fade. A solution containing all the above reagents except aluminium is  used for setting the instrument blank at zero.

Sample Treatment in the Absence of Fluoride and Phosphate

When the sample is known to be free of both fluoride and phosphate, the following procedure can be used. Transfer 12.5 ml of sample into a porcelain casserole, add several drops of methyl orange indicator, and titrate with 0.02N sulfuric acid to a faint pink color. Record the milliliters of acid added.

Into each to two 25 ml volumetric flasks add 12.5 ml of sample (70 to 75F) and the same amount of 0.02N sulfuric acid as determined above plus 0.5 ml of excess acid.

To one of the samples add 0.5 of 0.01MEDTA. This solution will be used as the instrument blank and adjusts for any color and turbidity present in the sample. Beyond this point each flask is treated in an identical fashion. Add 0.5-ml ascorbic acid solution, 5.0 ml of buffer, and mix. Then add 2.5 ml (accurately with a pipet) of Eriochrome Cyanine R dye indicator and dilute to the 25-ml mark with distilled water. Transfer each solution to a photometer cell.

The instrument is adjusted to zero absorbance with the EDTA blank. Read the sample absorbance of each solution between 5 and 15 minutes.

Modification when phosphates are present

To a 50 ml sample in a flask, add 0.9 ml of 6.0N sulfuric acid. Place the flask on a hot plate and boil gently for 90 minutes while maintaining the sample volume near the 15 ml mark by adding distilled water as needed. Allow the sample to cool to room temperature. Adjust the pH of the solution to 4.3 to 4.5 as measured with a pH meter, with 1.0N sodium hydroxide. Bring the  solution volume back up to 50 ml by adding distilled water, mix and use 12.5 ml portion for the aluminium test.

Run a blank in the same manner as described above; use 50 ml of distilled water and 0.9 ml of  6.0N sulfuric acid. This blank is used to set the instrument at zero absorbance before reading the sample.